In the production of integrated circuits utilizing a III/V semiconductor material, such as gallium arsenide, GaAs, for radar applications, it is often desirable to fabricate holes through the semiconductor chip. These holes in the semiconductor, called "vias", provide connections to electrical ground on the back of the chip. Vias eliminated almost all of the parasitic inductance associated with bond wires to ground. Vias can be particularly important to a good performance of large power transistors. It is also desirable to etch sharply defined, deep, flat-bottomed recess-tubs in III/V semiconductors, capable of containing a variety of electronic elements. Additionally, vias can be etched at the bottom of previously etched tubs.
One method of fabricating such vias and recesses, involves wet chemical etching. In this approach, a suitably masked wafer is immersed in a bath of etching solution. The etchant then eats through the exposed areas of the wafer. Good yield for this process is critically dependent on the etching characteristics of the chemical bath. The bath must etch smoothly and uniformly across the wafer. The etchant should be compatible with an etch mask that is easily applied to the wafer and easily removed.
Schwarzenberger, in U.S. Pat. No. 3,158,517, formed recesses in semiconductor bodies by applying an etching solution, having a pH of about 6, and consisting of 2 ml. of 30% H.sub.2 O.sub.2, 1,000 ml. of H.sub.2 O, and 100 g. of NH.sub.4 F, through selected areas of photo-resist lacquer. Y. Mori et al., in J. Electrochem. Soc., Vol. 125, No. 9, pp. 1510-1514, 1978, surveyed etching GaAs wafers using various mixtures of 85 wt.% H.sub.3 PO.sub.4, 30 wt.% H.sub.2 O.sub.2 and H.sub.2 O. Kuhn-kuhnenfeld et al., in U.S. Pat. No. 4,100,014, taught etching GaAs semiconductor wafers in a bath of 1 to 30 wt.% of HF, 2 to 30 wt.% of H.sub.2 O.sub.2, 1 to 75 wt.% of H.sub.2 SO.sub.4 and 15 to 95 wt.% of H.sub.2 O. While these etching solutions are effective in varying degrees, gas is usually evolved during dissolution of the GaAs. The bubbles of gas in the etch solution are a major point of difficulty, since they can be as large as the mask openings and block the flow of etchant to the wafer. What is needed then is an etching solution that will dissolve III/V semiconductor material, such as GaAs, uniformly and smoothly without the evolution of undesirable gas.